1. Field of the Invention
This invention relates to a transformer for a lamp driving circuit of a liquid crystal display, and more particularly to a method of winding a coil of a transformer in an inverter of a liquid crystal display that is adapted to reduce a power loss.
2. Discussion of the Related Art
A liquid crystal display (LCD) of an active matrix driving system may use thin film transistors (TFTs) as switching devices to display a natural moving picture. Since LCDs can be formed into smaller devices than existing Brown tubes, they are used commonly in computer monitors and laptops, in office automation equipment such as copy machines, and in portable equipment such as cellular phones and pagers.
Such LCDs usually require light sources such as backlights since the LCDs do not emit light themselves. A typical backlight is driven with an inverter and consumes a great amount of power in the LCD devices. The inverter may be divided into a DC/DC converter and a DC/AC converter. The DC/DC converter generates a DC voltage using a pulse width modulation (PWM). The DC/AC converter functions to convert the voltage applied from the DC/DC converter into an AC voltage that is sufficiently high to turn on a lamp.
However, the conventional inverter for an LCD consumes a large amount of power, which deteriorates the performance of a transformer that is contained in the DC/AC converter.
Referring to FIG. 1 and FIG. 2, a transformer for the inverter of the LCD includes a bobbin 1 wounded with a coil 2 and having a barrier rib 1a formed at every constant distance, and ferrite cores 4a and 4b introduced into the center of the bobbin 1.
The bobbin 1 provided with the barrier rib 1a is molded with a plastic. The ferrite cores 4a and 4b are mixed with a fine powder, such as iron oxide or manganese, that is responsible for guiding a magnetic flux. Each of these ferrite cores 4a and 4b is molded into an ‘E’ shape, and the centers thereof pass through the bobbin 1. The side walls of the ferrite cores 4a and 4b surround the side wall of the bobbin 1 wound with the coil 2. The coil 2 has a primary side and a secondary side wound at a different winding frequencies in accordance with predetermined winding ratios, and a current flows in the coil 2.
Each end of the bobbin 1 is provided with a lead pin 3. The coil 2 is wound within concave winding parts 5a, 5b and 5c between the barrier ribs 1a as shown in FIG. 3. The coil winding parts of the bobbin 1 wound with the coil 2 are wound with a tape. After the ferrite cores 4a and 4b are assembled into the bobbin 1, the ferrite cores 4a and 4b are surrounded by an adhesive tape. The winding parts 5a, 5b and 5c wound with the coil 2 correspond to a low voltage area, a middle voltage area and a high voltage area, respectively, while having the side walls of both ends of the bobbin 1 and the barrier ribs 1a therebetween. The low voltage area, the middle voltage area and the high voltage area each have a successively higher number of windings.
In a transformer of the inverter for the LCD, the winding arrangement of the coil 2 is shown in FIG. 4. In FIG. 4, the first round begins to be wound at the left side of the bottom within the left winding part Sa and then the next rounds are wound in a zigzag shape. After the 25th round is wound, the middle winding part 5b at the center thereof begins to be wound. Subsequently, in the same manner as the left winding part 5a, the 26th round begins to be wound at the left side of the bottom within the middle winding part 5b and the next rounds are wound in a zigzag shape. Thereafter, the right winding part 5c begins to be wound and then the next rounds are wound in the same manner. Finally, the last round Lst within the right winding part 5c is wound. The last round Lst is fed back into the left winding part Sa via a return line 2c that is connected to the lead pin 3.
Such a transformer of the inverter for the LCD has a capacitive impedance due to the barrier ribs 1a and the coil 2a going over the barrier ribs 1a and increases the capacitive impedance by the return wire 2c at the linear region in which the last winding round Lst is fed back. Since the large capacitive impedance deteriorates the efficiency of the transformer, the inverter consumes a great amount of power. In reality, when an output voltage of the secondary side of the conventional transformer is measured after determining a winding ratio of the primary side to the secondary side thereof in accordance with a voltage build-up rate, it has approximately 70% of the output voltage predicted by the winding ratio.